1. Field of the Invention
The present invention relates to atmospheric modeling and prediction and more particularly to a system and method employing acoustic sensing for profiling wind, temperature and turbulence.
2. Description of the Related Art
Currently, one of the biggest gaps in global weather monitoring, and therefore in the ability to predict weather, is wind data. Satellites do fairly well at temperature and water vapor content, but can only estimate winds by tracking clouds in photographs. This does not work in clear weather and it does not reveal winds below a cloud deck. Balloon-borne radiosondes measure winds aloft quite well, but only near the places where they are launched, and (usually) only twice a day.
The most useful form of wind monitoring would cover many locations, provide wind speed and direction at all altitudes above each location, and provide the information frequently. Because large transport planes routinely fly over much of the globe, they are a potentially useful tool for making such measurements. Some proposals call for equipping airliners with Doppler lidar or radar capable of measuring winds at many altitudes, even in clear air. However, commercial aircraft operators may be unwilling to accept the cost in weight, power, volume, maintenance, and supplemental FAA certification for carrying lidar or modifying radar.
An alternative is to place ground-based radar wind profilers such as Next Generation Weather Radar (NEXRAD) at many locations. The National Oceanic and Atmospheric Administration (NOAA) has deployed some of these within the US. Regrettably, these profilers are expensive and quite large. Because they are expensive, they are deployed sparsely, so there are important gaps in coverage, e.g. mountainous regions of the US. Because they are also large, they are unsuited to installation on the buoys currently deployed by NOAA at several locations in the oceans. Therefore, weather buoys measure only surface winds, not the higher altitude winds that drive weather.
Another alternative is to place Sound Detection and Ranging (SODAR) profilers at many locations. These devices emit a loud pulse of sound, then measure the weak signal scattered backward by air. The amplitude and Doppler shift of the return signal can reveal winds. The altitude range of SODAR is less than a kilometer as demonstrated by data provided at http://www.sodar.com/about_sodar.htm. Though smaller and cheaper than NEXRAD, the devices are still too large and costly for widespread deployment or for use on buoys. SODAR devices also lead to complaints by people living nearby, since the sound pulse is audible. A related technology is acoustic tomography using time of flight from pulsed active emitters to an array of microphones. This has been used by some foreign groups to estimate winds within a few hundred meters of the ground (see K. Arnold, A. Ziemann, A. Raabe, “Tomographic Monitoring of Wind and Temperature at Different Heights Above the Ground”, acta acustica, Vol. 87 (2001) 703-708), but this method has the same altitude limitations as SODAR.
Another alternative is to launch more radiosondes (weather balloons). These typically get good wind and temperature measurements from the surface to the top of the troposphere. However, each is used only once, so using more of them would increase the $36 million the National Weather Service already spends each year on radiosondes. The cost is increased more if the radiosondes must be released from ships to get ocean coverage: ship operations cost so much that NOAA recently stopped funding a meteorology ship in the North Pacific, even though this action strongly reduced the quality of forecasts on the US west coast.
It is therefore desirable to provide an affordable way to gather frequent wind vector data over oceans or other remote areas without large payloads on aircraft or buoys.
It is also desirable to provide a means for detecting turbulence. Turbulence refers to localized wind variations that are not part of large-scale movement of bodies of air. Turbulence is notoriously hard to detect in clear air aside from direct flight encounters by aircraft. Existing methods mostly have short range in clear air, and are therefore used only near airports where wind shear is a hazard. It is desirable to provide a solution for detecting turbulence over larger areas.